Outer surface proteins (Osps) represent the primary site for interactions between host and bacterial components. The array of proteins on the Borrelia burgdorferi (Bb) outer surface has been shown to vary with the infectious cycle. We are developing methods for studying the regulation of outer surface protein gene expression both in vivo and in culture conditions that mimic in vivo stimuli. We have constructed fusions between the promoters of several osp genes and a reporter (E. coli lacZ), in order to study regulation in E. coli. Total genomic and plasmid DNA libraries were constructed, transformed into osp-lac fusion containing strains, and individual plasmids were assayed for effects on fusion expression. Potential regulatory regions for the infectious cycle-regulated ospC gene were sequenced and compared. We have also measured RNA levels derived from specific genes in various growth conditions.